Information device

ABSTRACT

A compact information device in which a shortened unlatch time is enabled so such device can transition from a power cutoff mode to a power supply mode to resume supplying power to internal circuits as a result of a latch circuit unlatching a control signal. Embodiments of such a device, which may be a printer, includes a CPU, a power switch, a capacitor, a switch circuit, an unlatch signal generating unit, and a switch control signal generator, which may include a latch circuit, and a potential difference detection circuit. The latch circuit and potential difference detection circuit operate with voltage Vin2. The CPU outputs a start latch signal VLT when the operating mode changes from the power supply mode to the power cutoff mode. When the power switch is turned off, the potential difference detection circuit outputs an unlatch signal VRST. When the unlatch signal VRST is input, the latch circuit unlatches the control signal.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of, and claims priority under 35U.S.C. §120 on, U.S. application Ser. No. 12/915,498, filed Oct. 29,2010. This application also claims priority under 35 U.S.C. §119 onJapanese patent application no. 2009-259647, filed Nov. 13, 2009. Thecontent of each such related application is incorporated by referenceherein in its entirety.

BACKGROUND

1. Field of Invention

The present invention relates to an information device, and moreparticularly to enabling more rapid resumption of printing operations ofan information device having an off mode in which limited power isconsumed, when such device is turned on.

2. Description of Related Art

Some printers and other so-called small information devices operateusing DC power supplied from an externally connected power supplydevice. In order to reduce power consumption, demand has grown forinformation devices that cut off the supply of DC power to internalcircuitry including the CPU, or more particularly to components withrelatively high power consumption, when the power switch is on but theinformation device is not used for an extended time and the mainfunctions are not needed.

The European Union's Framework Directive on Eco-Design of Energy-UsingProducts requires that power consumption be limited to 0.5 W or lesswhen in the off mode, that is, an operating state in which a device isconnected to a commercial power source but no device functions are used.

Information devices with an off mode may have a switch circuit thatinterrupts the supply of DC power to internal circuits, and a latchcircuit that holds the switch circuit open, as a power supply cutoffswitch for cutting off the supply of DC power to internal circuits. Suchinformation devices may also have a smoothing capacitor for stabilizingthe supply voltage, and a discharge circuit including a dischargeresistance for discharging any residual charge in the smoothingcapacitor as necessary. Note that a capacitor with relatively highcapacitance is used as the smoothing capacitor.

When the supply of DC power to the internal circuitry is cut off in suchan information device, a start latch signal is input from the CPU to thelatch circuit and the latch circuit latches the start latch signal, forexample. While the start latch signal is latched, the latch circuitoutputs a switch control signal to the switch circuit to hold the switchcircuit open. While this switch control signal is input, that is, whilethe latch circuit latches the start latch signal, the switch circuitremains open and interrupts the supply of DC power to the internalcircuitry. In order to resume supply of DC power to the internalcircuits from this mode, the power switch of the information device mustfirst be turned off and then turned on again. This is described below.

More specifically, if the power switch of the information device isturned off when the switch circuit is open and the supply of DC power tointernal circuits is interrupted, the residual charge in the smoothingcapacitor is discharged by the operation of the discharge circuit. Whenthe voltage between the end terminals of the smoothing capacitor dropsbelow the operating voltage of the latch circuit as a result of theresidual charge being discharged, the latch circuit unlatches the startlatch signal. When the start latch signal is unlatched, the switchcircuit closes, and when the power switch turns on again DC power issupplied to internal circuits including the CPU.

See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2006-166561and Japanese Unexamined Patent Appl. Pub. JP-A-2005-312162.

This means that the user of the information device may turn the powerswitch off and then immediately turn the power switch on again in orderto resume supplying DC power to the internal circuits as describedabove. When this happens, however, the residual charge in the smoothingcapacitor may not be sufficiently discharged and the latch circuit maynot unlatch. As a result, DC power cannot be supplied to the internalcircuits and the internal circuits cannot operate because the switchcircuit remains open even though the power switch was turned on. Toeliminate such problems, the time required to unlatch (the unlatch time)may conceivably be shortened by using a resistor with the lowestpossible resistance and the greatest possible allowable loss as thedischarge resistor disposed in the discharge circuit.

However, because the external dimensions of resistors with suchcharacteristics are relatively large, using such a resistor as adischarge resistor leads to an increase in the size of the informationdevice. Resistors with such characteristics are therefore not desirablefor use in such information devices.

SUMMARY OF INVENTION

An information device according to the present invention resumessupplying DC power to internal circuits by unlatching a start latchsignal, following interruption of the supply of DC power to internalcircuits including a CPU. The unlatch time is advantageously shortenedto lessen work disruption, without adding any undesirable components.

One aspect of the invention is embodied in an information device havinga plurality of operating modes including a first operating mode in whichDC power is supplied and a second operating mode in which supply of DCpower is interrupted. The information device comprises a control unitthat outputs a start latch signal when the operating mode changes fromthe first operating mode to the second operating mode; a power switchhaving an input terminal connected to an external power supply deviceand an output terminal that changes a connection state between aconnection on state and a connection off state; a capacitor having afirst terminal connected to the output terminal of the power switch anda second terminal connected to ground; a switch circuit, having a firstterminal connected to the output terminal of the power switch and asecond terminal directly or indirectly connected to the control unit,that operates in response to a switch control signal, the switch circuitbeing in an open position in which the supply of DC power to the controlunit is cut off when the switch control signal is input to the switchcircuit and in a closed position when the switch control signal is notinput to the switch circuit; an unlatch signal generating unit thatdetects the connection state of the power switch, and when theconnection state is off, outputs an unlatch signal; and a switch controlsignal generator configured to receive the start latch signal and theunlatch signal, and further configured to selectively generate theswitch control signal based on at least whether or not the unlatchsignal is received. The control unit operates by DC power suppliedthrough the second terminal of the switch circuit, and the unlatchsignal generating unit and at least a part of the switch control signalgenerator operate by DC power supplied through the first terminal of theswitch circuit.

None of the various connections need be direct; other circuit componentssuch as resistors may be connected between the stated components.

The information device according to this aspect of the invention canoperate as described below when the operating mode is the secondoperating mode and the switch circuit is open.

(1) The operating unit of the power switch changes from the firstposition to the second position to disconnect its input and outputterminals and render the power switch in the off connection state.

(2) The unlatch signal generating unit detects when the power switchturns off, and when it does, outputs an unlatch signal to the switchcontrol signal generator.

(3) A latch circuit in the switch control signal generator unlatches thestart latch signal when the unlatch signal is input.

As a result, the latch circuit can unlatch the signal even when theresidual charge in the capacitor is not sufficiently discharged and thevoltage between the terminals of the capacitor is below the operatingvoltage of the latch circuit. There is, therefore, no need to use aresistor with low resistance and high allowable loss as the dischargeresistor in order to shorten the unlatch time as described above. Morespecifically, this aspect of the invention enables reducing the size ofthe information device while also shortening the unlatch time.

Note that as used herein the “connection on state” (or simply that thepower switch is on) means at least that the position of the operatingunit of the power switch is the closed position. In addition, the“connection state off state” (or simply that the power switch is off)means at least that the position of the operating unit of the powerswitch is the open position.

According to another aspect, the unlatch signal generating unit includesa potential difference detection circuit that detects the potentialdifference between the input terminal and output terminal of the powerswitch, and when the potential difference is greater than or equal to aspecified value determines that the connection state is off and outputsthe unlatch signal to the switch control signal generator.

The behavior of the information device according to this aspect of theinvention is described below.

(1) When the device is in the second operating mode and the switchcircuit is open, the operating unit of the power switch changes from thefirst position to the second position, and the connection of the inputterminal and output terminal turns off.

(2) The voltage between the terminals of the capacitor drops, and apotential difference results between the input terminal and the outputterminal of the power switch.

(3) The potential difference detection circuit detects this potentialdifference.

(4) When this potential difference is greater than or equal to aspecific value, the power switch is determined to have turned off andthe unlatch signal is output to the switch control signal generator.

According to another aspect, a resistor is connected between the firstterminal of the capacitor and a power supply terminal of the latchcircuit, and the resistance of the resistor is set so that residualcharge in the capacitor is discharged within a specified time when theconnection state of the power switch turns off.

When the resistance of the resistor is high, current consumption by theresistor decreases. As a result, the drop in the voltage between theterminals of the capacitor is delayed (the discharge time of theresidual charge in the capacitor increases) when the power switch turnsoff. A relatively long time is therefore required for the potentialdifference detection circuit to detect that a potential differenceexceeding a specified level is present between the input terminal of thepower switch and the output terminal to which the smoothing capacitor isconnected and the unlatch signal is output.

On the other hand, if the resistance of the resistor is low, currentflows to the resistor only when the device is in the second operatingmode. As a result, the discharge time of the residual charge in thecapacitor can be shortened when the power switch turns off withoutaffecting the first operating mode.

In addition, this aspect enables setting the resistance of the resistorwith consideration for the discharge time of the capacitor. As a result,the potential difference detection time when the power switch turns off,that is, the time required for a potential difference greater than orequal to a specified level to occur between the input terminal andoutput terminal of the power switch, can be suitably set.

The time required for the potential difference between the inputterminal and output terminal of the power switch to rise above aspecified level when the power switch turns off can be shortened themost by setting the resistance of the resistor to the maximum currentlevel allowed in the second operating mode. The current level allowed inthe second operating mode is preferably as low as possible in order tominimize power consumption, but depending on product functions mayconceivably be desirably increased to shorten the potential differencedetection time. The foregoing maximum current may also be determined byvarious laws, regulations, or product specifications.

According to another aspect, the control unit outputs an unlatchprevention signal to the switch control signal generator while theoperating mode is set to the first operating mode, and stops outputtingthe unlatch prevention signal when the operating mode changes from thefirst operating mode to the second operating mode; and the switchcontrol signal generator further comprises an unlatch prevention circuitthat prohibits the input of the unlatch signal to the latch circuitwhile the unlatch prevention signal is input.

When the operating mode is set to the first operating mode, thepotential difference between the input terminal and output terminal ofthe power switch may rise above the specific value even when the powerswitch is on due, for example, to increased contact resistance caused bydeterioration of the contacts resulting from sulfides or siloxanedeposits on the contact members of the power switch. In such situations,however, the potential difference detection circuit outputs the unlatchsignal in the same way as when the power switch is off. As a result, anundesirable unlatch signal may be input to the switch control signalgenerator. Input of an undesirable unlatch signal and resulting problemsare prevented in the information device according to this fourth aspectof the invention by means of the unlatch prevention circuit disposed inthe switch control signal generator.

According to another aspect, the switch control signal generator furthercomprises an OR circuit that receives as an input a state signal fromthe latch circuit indicating that the start latch signal is latched andthe unlatch signal output from the unlatch prevention circuit, and thatoutputs the switch control signal.

When the power switch turns off while the operating mode is set to thesecond operating mode in the information device according to thisaspect, the latch circuit unlatches due to the unlatch signal and theswitch circuit closes. As a result, even though the power switch is off,current could flow from the capacitor to the control unit and thecontrol unit may operate in error. However, because the switch controlsignal generator in the information device has an OR circuit, the switchcontrol signal generator continues to output a switch control signal tointerrupt supply of DC power to the control unit as a result of theunlatch signal output from the unlatch prevention circuit even if thepower switch turns off while the operating mode is set to the secondoperating mode, and the switch circuit can be held open. Control unitoperating errors can therefore be prevented.

According to another aspect, the control unit includes an errordetection unit that detects a specific error in the information device;and the error detection unit outputs the start latch signal to theswitch control signal generator when the error is detected.

When the error detection unit detects a specific error in an informationdevice according to this sixth aspect of the invention, the switchcontrol signal generator outputs a switch control signal to cutoffsupply of DC power to the control unit as a result of the latch circuitlatching the start latch signal. Therefore, when a specific error isdetected by the error detection unit, the switch circuit opens andsupply of DC power to the control unit can be stopped. As a result,problems can be prevented from becoming greater when a problem occurs inthe control unit. Note that errors detected by the error detectioncircuit include, for example, overvoltage errors, low voltage errors,short circuit errors, and other errors from which recovery is possibleby turning the power off and then on again.

An information device according to any of the foregoing configurationsmay comprise a printer.

Because printers are relatively small information devices, these aspectsof the invention are particularly well suited thereto.

In addition to hardware embodiments of an information device asdescribed above, the invention can also be rendered as a control methodfor an information device.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically describes the configuration of a computer system100 that includes an information device as a first embodiment of theinvention.

FIG. 2 schematically describes the configuration of a power supplycutoff circuit used in a printer 10 according to a first embodiment ofthe invention.

FIG. 3 is a timing chart describing the operation of the power supplycutoff circuit in the printer 10 according to the first embodiment ofthe invention.

FIG. 4 schematically describes the configuration of a power supplycutoff circuit in a printer according to the related art.

FIG. 5 is a timing chart describing the operation of the power supplycutoff circuit in a printer according to the related art.

FIG. 6 schematically describes the configuration of a power supplycutoff circuit in a printer according to a second embodiment of theinvention.

FIG. 7 is a timing chart describing the operation of the power supplycutoff circuit in the printer 10 according to the second embodiment ofthe invention.

FIG. 8 schematically describes the configuration of a power supplycutoff circuit in a printer according to a third embodiment of theinvention.

FIG. 9 is a timing chart describing the operation of the power supplycutoff circuit in the printer 10 according to the third embodiment ofthe invention.

FIG. 10 schematically describes the configuration of a power supplycutoff circuit in a printer according to another embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are described below withreference to the accompanying figures.

A. Embodiment 1 A1. System Configuration

FIG. 1 schematically describes the configuration of a computer system100 that includes an information device as a first embodiment of theinvention. This computer system 100 includes a personal computer PC anda printer 10 as an example of an information device according to theinvention.

An AC adaptor 20 is connected to the printer 10 through a power cable22. The AC adaptor 20 is connected to a commercial power supply throughpower cable 24. The AC adaptor 20 converts the AC power supplied fromthe commercial power source to DC power, and supplies DC power to theprinter 10. The AC adaptor 20 is a power supply device.

A personal computer PC is connected to the printer 10 through a printercable 30. A keyboard KB, mouse MS, and display DP are connected to thepersonal computer PC. The printer 10 prints when a print job is receivedfrom the personal computer PC.

To suppress power consumption, the printer 10 according to thisembodiment of the invention has two operating modes, a power supply modethat is the normal operating mode in which DC power supplied from the ACadaptor 20 is supplied to internal circuits including a CPU, and a powercutoff mode (off mode) in which the supply of DC power to internalcircuits is stopped.

When print jobs are not sent from the personal computer PC to theprinter 10 for a long time, the operating mode changes from the powersupply mode to the power cutoff mode. The printer 10 user can desirablyset the delay time for the operating mode to change from the powersupply mode to the power cutoff mode.

A power supply cutoff circuit interrupts the supply of DC power to theinternal circuits when the operating mode changes from the power supplymode to the power cutoff mode. This power supply cutoff circuit isdescribed in detail below.

Note that the power supply mode includes a normal mode, which is theoperating mode used for printing and in which power consumption isrelatively high, and an energy conservation mode, which is an operatingmode with relatively low power consumption while waiting to receive thenext print job for a specific time after one print job ends.

A2. Power Supply Cutoff Circuit Configuration

FIG. 2 describes the configuration of the power supply cutoff circuitdisposed in the printer 10 according to the first embodiment of theinvention. As shown in the figure, the power supply cutoff circuitincludes a tactile switch 12, smoothing capacitor 13, resistor 13R,switch circuit 14, switch control signal generator 15, and potentialdifference detection circuit 16. The switch control signal generator 15includes a latch circuit 15 a. The latch circuit 15 a goes to ground.

The tactile switch 12 has an input terminal 12 a, output terminal 12 b,and operating unit 12 c. The tactile switch 12 is a power switch thatopens and closes by changing the position of the operating unit 12 cbetween a first position where the connection state of the inputterminal 12 a and output terminal 12 b (also referred to below as simplythe connection state) is on, and a second position where the connectionstate is off. Unless the tactile switch 12 is operated, the tactileswitch 12 holds the position and the connection state of the operatingunit 12 c.

The input terminal 12 a is connected to the AC adapter 20.

The smoothing capacitor 13 has a first terminal 13 a and a secondterminal 13 b. The first terminal 13 a is connected to the outputterminal 12 b of the tactile switch 12, and the second terminal 13 bgoes to ground. The smoothing capacitor 13 is a capacitor forstabilizing voltage Vin2. A capacitor with relatively high capacitanceis used as this smoothing capacitor 13.

Resistor 13R is connected between the first terminal 13 a of thesmoothing capacitor 13 and the power supply pin of the latch circuit 15a. When the tactile switch 12 turns off, this resistor 13R is used todischarge residual charge in the smoothing capacitor 13 through thelatch circuit 15 a. The resistance of this resistor 13R is set accordingto the power consumption and discharge time required to discharge theresidual charge of the smoothing capacitor 13. In this embodiment of theinvention the resistance of the resistor 13R is set so that thedischarge time is as short as possible while power consumption when theresidual charge is discharged is 0.5 W or less (the upper limit of theEco-Design Directive).

The switch circuit 14 is connected to the output terminal 12 b of thetactile switch 12. Based on the switch control signal VSC input from theswitch control signal generator 15, the switch circuit 14 switchesbetween supplying and interrupting the supply of DC power to internalcircuits including a CPU 17 and error detection circuit 18. When aswitch control signal VSC to interrupt the supply of DC power tointernal circuits is input, the switch circuit 14 opens. When a switchcontrol signal VSC to interrupt the supply of DC power to internalcircuits is not input, the switch circuit 14 closes. DC power passingthrough the switch circuit 14 is voltage converted (voltage Vcc) by aDC/DC converter not shown, and supplied to the CPU 17 and errordetection circuit 18. The switch circuit 14 includes a FET (Field Effecttransistor), for example.

Based on a start latch signal VLT input from the CPU 17 or errordetection circuit 18, and a unlatch signal VRST input from the potentialdifference detection circuit 16, the switch control signal generator 15generates switch control signal VSC and outputs this switch controlsignal VSC to the switch circuit 14. In this embodiment of theinvention, when the start latch signal VLT is input to the latch circuit15 a and the start latch signal VLT is latched by the latch circuit 15a, the switch control signal generator 15 outputs the switch controlsignal VSC to interrupt the supply of DC power to internal circuits. Thelatch circuit 15 a unlatches when the unlatch signal VRST is input.

When the start latch signal VLT is not latched by the latch circuit 15a, the switch control signal generator 15 does not output the switchcontrol signal VSC to interrupt supply of DC power to internal circuits.

Note that the latch circuit 15 a operates using the voltage Vin2 inputfrom the AC adaptor 20 through the resistor 13R downstream from thetactile switch 12.

The potential difference detection circuit 16 detects the potentialdifference ΔV (Vin1-Vin2) between the input terminal 12 a and outputterminal 12 b of the tactile switch 12, and if the potential differencebetween the terminals is greater than or equal to a specified value(such as greater than 1 V), outputs the unlatch signal VRST to theswitch control signal generator 15. More specifically, the potentialdifference detection circuit 16 outputs the unlatch signal VRST when thetactile switch 12 is off and the potential difference ΔV between theinput terminal 12 a and output terminal 12 b of the tactile switch 12 isgreater than or equal to a specified value.

To change the operating mode from the power supply mode to the powercutoff mode, the CPU 17 outputs the start latch signal VLT to thepotential difference detection circuit 16.

When the operating mode of the printer 10 is the power supply mode, theerror detection circuit 18 looks for specific errors in the printer 10,and if such an error is detected outputs the start latch signal VLT tothe switch control signal generator 15. Errors detected by the errordetection circuit 18 include, for example, overvoltage errors, lowvoltage errors, and short circuit errors. The CPU 17 and error detectioncircuit 18 are control units as described in the summary of theinvention.

A3. Operation of the Power Supply Cutoff Circuit

FIG. 3 is a timing chart describing the operation of the power supplycutoff circuit in the printer 10 according to the first embodiment ofthe invention.

When the position of the operating unit 12 c of the tactile switch 12changes from the second position to the first position at time t0 whenthe tactile switch 12 is off, the tactile switch 12 turns on, voltageVin2 rises, and the operating mode of the printer 10 goes to the powersupply mode.

The CPU 17 then watches a timer, and at time t1 when a specified timehas passed without a print job being sent from the personal computer PC,outputs the start latch signal VLT to change the operating mode of theprinter 10 from the power supply mode to the power cutoff mode. When thelatch circuit 15 a latches this start latch signal VLT (latch state:on), the switch control signal generator 15 outputs the switch controlsignal VSC that interrupts supply of DC power to internal circuits tothe switch circuit 14. When this switch control signal VSC is input tothe switch circuit 14, the switch circuit 14 opens. After the switchcircuit 14 opens and supply of DC power to internal circuits isinterrupted, the operating mode of the printer 10 remains in the powercutoff mode until the tactile switch 12 is turned off and then on again.

In order to reset the operating mode of the printer 10 from the powercutoff mode to the power supply mode, at time t3 the position of theoperating unit 12 c of the tactile switch 12 changes from the firstposition to the second position and the tactile switch 12 turns off.When the tactile switch 12 turns off, the residual charge of thesmoothing capacitor 13 is discharged through resistor 13R and the latchcircuit 15 a, and the voltage between the terminals of the smoothingcapacitor 13 (voltage Vin2) drops. As a result, a potential differenceis produced between the input terminal 12 a and output terminal 12 b ofthe tactile switch 12.

If the potential difference detection circuit 16 detects that thepotential difference between the input terminal 12 a and output terminal12 b of the tactile switch 12 is greater than or equal to a specifiedvalue, it outputs the unlatch signal VRST to the switch control signalgenerator 15.

When this unlatch signal VRST is input to the latch circuit 15 a and thelatch circuit 15 a unlatches (latch state: off), the switch controlsignal generator 15 stops outputting the switch control signal VSC thatinterrupts supply of DC power to internal circuits. When input of theswitch control signal VSC that interrupts supply of DC power to internalcircuits stops, the switch circuit 14 closes.

At time t4 when the position of the operating unit 12 c of the tactileswitch 12 changes from the second position to the first position and thetactile switch 12 turns on, the operating mode of the printer 10 returnsto the power supply mode.

Note that in this embodiment of the invention the time between time t3and time t4 described above is, for example, approximately 0.5 second,which as the switching time of the operating unit 12 c of the tactileswitch 12 is relatively short. Therefore, while the residual charge ofthe smoothing capacitor 13 is discharged through resistor 13R and latchcircuit 15 a during the period from time t3 to time t4, voltage Vin2does not drop below the operating voltage of the latch circuit 15 a bytime t4.

As described above, when the operating mode of the printer 10 is thepower supply mode, the error detection circuit 18 looks for specificerrors inside the printer 10, and outputs the start latch signal VLT tothe switch control signal generator 15 when it detects an error. In thissituation, while not shown in the figure, the power supply cutoffcircuit operates in the same way as when the start latch signal VLT isoutput from the CPU 17 at time t1, and the operating mode of the printer10 changes from the power supply mode to the power cutoff mode.

A4. Effect

To describe the effect of the printer 10 according to the firstembodiment of the invention having the power supply cutoff circuitdescribed above, a power supply cutoff circuit used in a printeraccording to the related art that has a power supply mode and powercutoff mode as operating modes is described below.

FIG. 4 schematically describes the power supply cutoff circuit used in aprinter according to the related art. As will be understood by comparingFIG. 4 and FIG. 2, this power supply cutoff circuit differs from thepower supply cutoff circuit used in the printer 10 according to thefirst embodiment of the invention in that it has a discharge circuitincluding a discharge resistor R and switch circuit SW, does not have apotential difference detection circuit 16, and does not have an errordetection circuit 18. Note that the discharge circuit is a circuit thatoperates when the tactile switch 12 switches from on to off, anddischarges the residual charge of the smoothing capacitor 13.

FIG. 5 is a timing chart describing the operation of the power supplycutoff circuit in a printer according to the related art. The operationfrom time t0 to time t3 is the same as the operation of the power supplycutoff circuit in the printer 10 according to the first embodiment ofthe invention (see FIG. 3).

With the power supply cutoff circuit used in a printer according to therelated art, the discharge circuit operates and the residual charge ofthe smoothing capacitor 13 is discharged when the tactile switch 12turns off at time t3. When the voltage (voltage Vin2) between theterminals of the smoothing capacitor 13 then drops below the operatingvoltage of the latch circuit 15Ra at time t5, the latch circuit 15Raunlatches (latch state: off). When the latch circuit 15Ra unlatches, theswitch circuit 14 closes. When the tactile switch 12 then turns on againat time t6, DC power is supplied to the internal circuits.

More specifically, with the power supply cutoff circuit used in aprinter according to the related art, the latch circuit 15Ra does notunlatch and the switch circuit 14 remains open until the voltage betweenthe terminals of the smoothing capacitor 13 goes below the operatingvoltage of the latch circuit 15Ra. Therefore, even if the tactile switch12 turns on at time t4 before the voltage between the terminals of thesmoothing capacitor 13 drops below the operating voltage of the latchcircuit 15Ra, the switch circuit 14 remains open and DC power cannot besupplied to internal circuits.

To solve this problem a resistor with the lowest possible resistance andthe greatest possible allowable loss may conceivably be used as thedischarge resistor R disposed in the discharge circuit to shorten theunlatch time of the latch circuit 15Ra. However, because the externaldimensions of resistors with such characteristics are relatively large,using such a resistor as the discharge resistor R leads to an increasein the size of the printer. Resistors with such characteristics aretherefore unsuitable for use in printers.

However, because the power supply cutoff circuit of the printer 10according to the first embodiment of the invention has a potentialdifference detection circuit 16, the latch circuit 15 a can be unlatchedby applying the unlatch signal VRST output from the potential differencedetection circuit 16. The printer 10 according to the first embodimentof the invention therefore does not need to use a resistor with lowresistance and high allowable loss as the discharge resistor R in orderto shorten the unlatch time. More specifically, the printer 10 accordingto the first embodiment of the invention enables both a small printer 10and shortening the unlatch time.

Furthermore, because the error detection circuit 18 of the printer 10according to the first embodiment of the invention can output the startlatch signal VLT and interrupt the supply of DC power to the internalcircuits when a specific error is detected, problems can be preventedfrom becoming bigger when a problem occurs in the internal circuits.

Furthermore, with the printer 10 according to the first embodiment ofthe invention, the resistance of the resistor 13R in the power supplycutoff circuit can be set with consideration for the discharge time andpower consumption when discharging the residual charge of the smoothingcapacitor 13. Therefore, by appropriately setting the operating currentof the latch circuit 15 a, that is, the current flowing to the resistor13R, when the operating mode is the power cutoff mode, the time until apotential difference greater than or equal to a specified value isproduced between the input terminal 12 a and output terminal 12 b of thetactile switch 12 (the time until the potential difference is detected)when the tactile switch 12 turns off can be set appropriately.

B. Embodiment 2

A printer according to the second embodiment of the invention has apower supply mode and a power cutoff mode as operating modes similarlyto the printer 10 according to the first embodiment of the invention.However, the configuration of the power supply cutoff circuit in theprinter according to the second embodiment of the invention differs fromthat of the printer 10 according to the first embodiment of theinvention. The configuration and operation of the power supply cutoffcircuit in a printer according to the second embodiment of the inventionis described next.

B1. Power Supply Cutoff Circuit Configuration

FIG. 6 schematically describes the configuration of the power supplycutoff circuit in a printer according to the second embodiment of theinvention. As will be known by comparing FIG. 6 and FIG. 2, the powersupply cutoff circuit of the printer according to the second embodimentof the invention has a switch control signal generator 15A instead ofthe switch control signal generator 15 used in the first embodiment.

This switch control signal generator 15A has an unlatch preventioncircuit 15 b in addition to a latch circuit 15 a. When the operatingmode of the printer 10 is the power supply mode, the CPU 17 outputs anunlatch prevention signal VPR to the unlatch prevention circuit 15 b,and when the operating mode changes from the power supply mode to thepower cutoff mode, stops output of the unlatch prevention signal VPR(see FIG. 7).

While the unlatch prevention signal VPR is input, the unlatch preventioncircuit 15 b prohibits output of the unlatch signal VRST from thepotential difference detection circuit 16 to the latch circuit 15 a.

The configuration of this embodiment is otherwise identical to theconfiguration of the first embodiment.

B2. Operation of the Power Supply Cutoff Circuit

FIG. 7 is a timing chart of the operation of the power supply cutoffcircuit in a printer according to the second embodiment of theinvention. The operation of the power supply cutoff circuit in thesecond embodiment is substantially the same as in the first embodiment.However, because the power supply cutoff circuit of the secondembodiment also has an unlatch prevention circuit 15 b, the unlatchsignal VRST output from the potential difference detection circuit 16 isnot input to the latch circuit 15 a while the operating mode is thepower supply mode.

When the operating mode is the power supply mode, a potential differencemay occur between the input terminal 12 a and output terminal 12 b ofthe tactile switch 12 even when the tactile switch 12 is on. Forexample, the potential difference may become greater than the specifiedvalue as indicated by the dot-dash line in FIG. 7 due to an increase incontact resistance resulting from deterioration of the contacts as aresult of sulfides or siloxane, for example, adhering to the contactmembers of the tactile switch 12. In this situation the potentialdifference detection circuit 16 outputs the unlatch signal VRST just asif the tactile switch 12 is off (see the unlatch signal VRST in FIG. 7).As a result, when an unlatch prevention circuit 15 b is not used,similarly to the switch control signal generator 15 in the firstembodiment, the unlatch signal VRST may be undesirably input to theswitch control signal generator 15A. However, by rendering an unlatchprevention circuit 15 b in the switch control signal generator 15A ofthe power supply cutoff circuit, the printer according to the secondembodiment of the invention can prevent problems caused by input of anundesirable unlatch signal.

As in a printer 10 according to the first embodiment of the invention,the printer according to the second embodiment of the invention canunlatch the latch circuit 15 a without using a discharge resistor R byapplying an unlatch signal VRST output from the potential differencedetection circuit 16 as described above, and thereby enables a smallprinter size and short unlatch time.

C. Embodiment 3

A printer according to the third embodiment of the invention has a powersupply mode and a power cutoff mode as operating modes similarly to theprinter 10 according to the second embodiment of the invention. However,the configuration of the power supply cutoff circuit in the printeraccording to this third embodiment differs from that of the printer 10according to the second embodiment of the invention. The configurationand operation of the power supply cutoff circuit in a printer accordingto the third embodiment of the invention is described next.

C1. Power Supply Cutoff Circuit Configuration

FIG. 8 schematically describes the configuration of the power supplycutoff circuit in a printer according to the third embodiment of theinvention. As will be known by comparing FIG. 8 and FIG. 6, the powersupply cutoff circuit of the printer according to the third embodimentof the invention has a switch control signal generator 15B instead ofthe switch control signal generator 15A used in the second embodiment.

This switch control signal generator 15B has an OR circuit 15 c inaddition to a latch circuit 15 a and unlatch prevention circuit 15 b. Astate signal VST output from the latch circuit 15 a indicating the latchstate of the latch circuit 15 a, and the unlatch signal VRST passedthrough the unlatch prevention circuit 15 b, are input to the OR circuit15 c.

Note that this state signal VST is the same signal as the switch controlsignal VSC in the second embodiment. The switch control signal generator15B outputs the signal output from the OR circuit 15 c as the switchcontrol signal VSC to the switch circuit 14.

More specifically, while the state signal VST output from the latchcircuit 15 a indicating that the start latch signal VLT is latched isinput to the OR circuit 15 c, or while the unlatch signal VRST is inputfrom the unlatch prevention circuit 15 b, the switch control signalgenerator 15B generates the switch control signal VSC to interrupt thesupply of DC power to internal circuits. In addition, when neither theunlatch signal VRST or the state signal VST indicating the start latchsignal VLT is latched is input to the OR circuit 15 c, the switchcontrol signal generator 15B does not generate the switch control signalVSC to interrupt the supply of DC power to internal circuits.

C2. Operation of the Power Supply Cutoff Circuit

FIG. 9 is a timing chart of the operation of the power supply cutoffcircuit in a printer according to the third embodiment of the invention.The operation from time t0 to time t3 is the same as the operation ofthe power supply cutoff circuit in a printer according to the secondembodiment of the invention (see FIG. 7).

With the power supply cutoff circuit used in a printer according to thesecond embodiment of the invention described above, when the operatingmode is the power cutoff mode and the tactile switch 12 turns off attime t3, the latch circuit 15 a unlatches due to the unlatch signal VRSTand the switch circuit 14 closes. As a result, even though the tactileswitch 12 is off, current could flow from the smoothing capacitor 13 tothe internal circuits including the CPU 17, and internal circuits mayoperate in error.

With the power supply cutoff circuit used in the printer according tothe third embodiment of the invention, however, such operating errors donot occur because the unlatch signal VRST is input to the OR circuit 15c. That is, when the tactile switch 12 turns off at time t3, and thepotential difference detection circuit 16 detects the potentialdifference between the input terminal 12 a and output terminal 12 b ofthe tactile switch 12 and outputs the unlatch signal VRST, this unlatchsignal VRST is input through the unlatch prevention circuit 15 b to theOR circuit 15 c. In addition, because this unlatch signal VRST is outputfrom the switch control signal generator 15B to the switch circuit 14 asthe switch control signal VSC to open the switch circuit 14, the switchcircuit 14 can be held open. Incorrect operation of the internalcircuits can therefore be prevented.

Note that if the unlatch prevention circuit 15 b is omitted from thepower supply cutoff circuit in the printer according to the thirdembodiment of the invention, the unlatch signal VRST is output from thepotential difference detection circuit 16 when the potential differencebetween the input terminal 12 a and output terminal 12 b of the tactileswitch 12 is greater than or equal to the specified value as indicatedby the dot-dash lines in FIG. 9. This unlatch signal VRST is also outputas the switch control signal VSC from the switch control signalgenerator 15B (OR circuit 15 c). As a result, the switch circuit 14 mayopen while the operating mode is set to the power supply mode. Thisproblem is prevented in this embodiment of the invention, however, bythe unlatch prevention circuit 15 b.

As in a printer 10 according to the first embodiment and the printeraccording to the second embodiment of the invention, the printeraccording to this third embodiment of the invention can unlatch thelatch circuit 15 a without using a discharge resistor R by applying anunlatch signal VRST output from the potential difference detectioncircuit 16 as described above, and thereby enables a small printer sizeand short unlatch time.

D. Other embodiments

Preferred embodiments of the invention are described, but the inventionis not so limited and can be varied in many ways without departing fromthe scope of the accompanying claims. Some examples of such variationsare described below.

D1. Variation 1

In the printer 10 according to the first embodiment above the powersupply cutoff circuit has a potential difference detection circuit 16,and the potential difference detection circuit 16 outputs an unlatchsignal VRST to the switch control signal generator 15, but the inventionis not so limited. For example, the power supply cutoff circuit may havean unlatch signal generator that detects the connection state of theinput terminal 12 a and output terminal 12 b of the tactile switch 12,and outputs unlatch signal VRST to the switch control signal generator15 if the connection state (tactile switch 12) is off.

FIG. 10 schematically describes the configuration of the power supplycutoff circuit in a printer according to this variation of theinvention. As will be known by comparing FIG. 10 and FIG. 2, the powersupply cutoff circuit in a printer according to this variation has asensor 16 s instead of the potential difference detection circuit 16 inthe first embodiment. This sensor 16 s detects if the tactile switch 12is turned off or on.

When the tactile switch 12 is off, the sensor 16 s outputs the unlatchsignal VRST to the switch control signal generator 15. When the tactileswitch 12 is on, the sensor 16 s does not output the unlatch signal VRSTto the switch control signal generator 15.

Note that the sensor 16 s may be any type of sensor that can detect ifthe tactile switch 12 is off or on, and an optical sensor or ultrasonicsensor, for example, may be used as the sensor 16 s. The same effect asthe printer 10 of the first embodiment can be achieved with the powersupply cutoff circuit according to this variation of the invention.

D2. Variation 2

In the embodiments described above the start latch signal VLT outputfrom the error detection circuit 18 is input to the latch circuit 15 aof the power supply cutoff circuit, but the start latch signal VLToutput from the error detection circuit 18 may be omitted.

D3. Variation 3

The embodiments described above use a tactile switch 12 as the powerswitch disposed to the power supply cutoff circuit, but the invention isnot so limited. The power switch used in the invention is generally apower switch that has an input terminal, an output terminal, and anoperating unit, and can be any type of switch of which the connectionstate changes as a result of changing the position of the operating unitbetween a first position in which the connection state of the inputterminal and the output terminal is on, and a second position in whichthe connection state is off, and which holds the position of theoperating unit and connection state unless the operating unit isoperated to change position. Examples of such switches include toggleswitches, slide switches, and rocker switches.

D4. Variation 4

The invention is described as applied to a printer in the foregoingembodiments, but can obviously also be applied to information devicesother than printers.

Although the present invention has been described in connection with thepreferred embodiments thereof with reference to the accompanyingdrawings, it is noted that various changes and modifications will beapparent to those skilled in the art in light of such disclosure. Anyand all such changes and/or modifications are intended to be within thescope of the present invention to the extent encompassed by any of theclaims of this application.

What is claimed is:
 1. An information device having a plurality ofoperating modes including a first operating mode in which DC power issupplied and a second operating mode in which supply of DC power isinterrupted, the information device comprising: a control unit thatoutputs a start latch signal when the operating mode changes from thefirst operating mode to the second operating mode; a power switch havingan input terminal connected to an external power supply device and anoutput terminal that changes a connection state between a connection onstate and a connection off state; a capacitor having a first terminalconnected to the output terminal of the power switch and a secondterminal connected to ground; a switch circuit, having a first terminalconnected to the output terminal of the power switch and a secondterminal directly or indirectly connected to the control unit, thatoperates in response to a switch control signal, the switch circuitbeing in an open position in which the supply of DC power to the controlunit is cut off when the switch control signal is input to the switchcircuit and in a closed position when the switch control signal is notinput to the switch circuit; an unlatch signal generating unit thatdetects the connection state of the power switch, and when theconnection state is off, outputs an unlatch signal; and a switch controlsignal generator configured to receive the start latch signal and theunlatch signal, and further configured to selectively generate theswitch control signal based on at least whether or not the unlatchsignal is received; wherein the control unit operates by DC powersupplied through the second terminal of the switch circuit, and theunlatch signal generating unit and at least a part of the switch controlsignal generator operate by DC power supplied through the first terminalof the switch circuit.